Embryonic history of the aortic arches in mammals. 413 



course, to a pulmonary artery. As in the preceding- embryo the 

 space between the fourth and sixth arches is considerably wider 

 than that between the third and the fourth. 



This embryo is of special interest as showing variation in the 

 order of development. The rudiments of the first and second arches 

 are more nearly complete than in the embryo shown in Fig-. 14, 

 Plate 23, of apparently about the same age, while the pulmonary 

 arches, which were not fully formed in that specimen, are complete 

 in this embryo. In both cases, the dorsal remnants of the first two 

 arches appear to be more persistent than the ventral ones. In 

 Fig, 16, Plate 24, we have five arches nearly complete. It is to 

 be noted in this connection that His and Tandler both show in 

 the human embryo five complete aortic arches existing at the 

 same time. 



Embryo of the twentieth day (No. 3, somewhat 

 older). This embryo represents a somewhat older stage in the 

 twentieth day. An external view is shown in Fig. 17, Plate 22. 

 Four well developed branchial pouches are present, and considerable 

 modification from the previous stages is shown in their contour. The 

 whole embryo is decidedly larger than that shown in Fig. 15, 

 Plate 22. 



Fig. 18, Plate 24 shows a reconstruction of the aortic arches 

 of the right side for this embryo. In this stage three complete 

 pairs of aortic arches are present; the third, fourth and sixth, and 

 remnants of the first two pairs. The vessels lie in the corresponding 

 visceral pouches. 



The vessel in the mandibular bar consists of a very slender branch 

 from the dorsal aorta, which after leaving the latter vessel, bends 

 toward the exterior and then takes a course ventralward through 

 the middle of the visceral arch. Opposite the dorsal end of the 

 cleft which separates the maxillary process of the first arch from 

 the remaining portion, this small vessel appears to break up into 

 sinuses which though disconnected, lie close together and can be 

 readily followed. These extend ventralward for a short distance, 

 and then take a general direction toward the anterior prolongation 

 of the truncus arteriosus. The remnants of the first arch upon 

 either side are essentially symmetrical. 



A vessel of somewhat larger diameter is present in the second 

 visceral arch of both sides. This artery also bends toward the 

 exterior upon leaving the dorsal aorta, and passes downward through 



